1. Field of the Invention
The present invention generally relates to a method for forming a joined or bound section of optical branch fibers and an apparatus therefor, and more particularly, to a method for forming the joined section of a plurality of optical fibers, constituting part of an optical coupler, and an apparatus therefor by which method the end faces of the joined section and a single optical trunk fiber can be connected with each other to complete the optical coupler.
2. Description of the Related Art
The formation of a joined section of branch fibers is a necessary process for fabricating an optical coupler in which a trunk fiber is coupled to the plurality of branch fibers. Optical couplers are disclosed in Japanese Patent Laid-Open Publication No. 57-205708 (hereinafter referred to as first prior art), Japanese Patent Laid-Open Publication No. 59-143119 (hereinafter referred to as second prior art), and Japanese Patent Laid-Open Publication No. 61-208010 (hereinafter referred to as third prior art).
According to the first prior art, each of free ends of the branch fibers is removed by shearing, cutting or grinding to define an undercut face adjacent the end face thereof. The undercut face is inclined at a predetermined angle to the axis of the fiber. Needless to say, the core of the fiber is exposed at both the undercut face and end face. Then, the undercut faces are brought in contact with each other to form the joined section of the branch fibers. Thereafter, the end face of the joined section and that of the trunk fiber are connected with each other. Therefore, unless a reflection layer consisting of a material such as aluminum, gold or silver is formed between the undercut faces confronting each other, light which is inputted to one of the branch fibers from the trunk fiber would leak into the other branch fiber through the undercut faces. Each of the branch fibers is required to be cut with high accuracy so that the undercut face is inclined precisely at a predetermined angle to the axis thereof. In addition, such highly accurate work must be carried out at a factory having special equipment to do so. For example, it is difficult to cut optical fibers with such high accuracy at a job site where they are to be installed. A high reproducibility of the optical coupler having a constant signal branching ratio cannot be accomplished due to the above-described situation.
According to the optical coupler of the second prior art, both ends of two branch fibers are inserted into a pair of fixtures serving as a die for deforming optical fibers and holding them. The two branch fibers are softened by heating them; then, the branch fibers are clamped by the fixtures. Consequently, the branch fibers are integrated with each other. The end face of the integrated section of the two branch fibers becomes approximately elliptical. Then, a trunk fiber is connected with the integrated section of the branch fibers thus formed by bringing the circular end face of the former into contact with the elliptical end face of the latter so as to form the optical coupler. The diameter of the circular end face of the trunk fiber is approximately equal to the minimum diameter of the elliptical end face of the joined section of the two branch fibers. That is, the sectional area of the trunk fiber is smaller than that of the joined section of branch fibers. According to this prior art, the trunk fiber is moved along the major axis of the elliptical end face of the joined section of the two branch fibers so that the trunk fiber can be connected with the two branch fibers at a position based on a desired signal branching. Further, the optical coupler can be fabricated with comparative ease. As is apparent from the above description, the signal branching ratio of the optical coupler is determined according to a position at which the trunk fiber is connected with the two branch fibers. Consequently, the optical coupler in accordance with the second prior art has necessarily the following disadvantage. A signal loss is not so large in the transmission of a signal from the trunk fiber to the branch fibers, but a signal loss is 50% or more in the transmission of a signal from the branch fibers to the trunk fiber. Thus, the optical coupler cannot be used to carry out a bidirectional transmission of a signal.
According to the optical coupler of the third prior art, similarly to the first prior art, each of free ends of the branch fibers is removed by shearing, cutting or grinding to define an undercut face adjacent the end face thereof. The undercut face is inclined at a predetermined angle to the axis of the fiber. Needless to say, the core of the fiber is exposed at both the undercut face and end face. Then, the undercut faces are brought in contact with each other to form the joined section of the branch fibers. Thereafter, the end face of the joined section and that of the trunk fiber are connected with each other. Therefore, unless a reflection layer consisting of a material such as aluminum, gold or silver is formed between the undercut faces confronting each other, light which is inputted to one of the branch fibers from the trunk fiber would leak into the other branch fiber through the undercut faces. Each of the branch fibers is required to be cut with high accuracy so that the undercut face is inclined precisely at a predetermined angle to the axis thereof. In addition, such highly accurate work must be carried out at a factory having special equipment to do so. For example, it is difficult to cut optical fibers with such high accuracy at a job site where they are to be installed. A high reproducibility of the optical coupler having a constant signal branching ratio cannot be accomplished due to the above-described situation. In the first prior art, the plurality of branch fibers are brought into contact with each other with a reflection layer interposed between the respective undercut faces thereof. According to the third prior art, cores of respective branch fibers and cladding thereof are joined to each other by means of fusing without interposing reflection layers between the respective undercut faces. Although this construction causes little signal loss to be produced when transmitting a signal, it is difficult to obtain a desired signal branching ratio because light permeates between branch fibers through the welded undercut faces.